Wireless metropolitan area networks (MAN) are networks implemented over an air interface for fixed, portable and mobile broadband access systems. Some Wireless MANs utilize orthogonal frequency division multiplexing (OFDM) for signaling between mobile terminals, and/or subscriber stations (SS), and base stations. OFDM is a form of multiplexing that distributes data over a number of carriers that have a very precise spacing in the frequency domain. The precise spacing of the carriers provides several benefits such a high spectral efficiency, resiliency to radio frequency interference and lower multi-path distortion. Due to its beneficial properties and superior performance in multi-path fading wireless channels, OFDM has been identified as a useful technique in the area of high data-rate wireless communication, for example wireless MANs. Orthogonal frequency division multiple access (OFDMA) is a multiple access technology that utilizes OFDM techniques.
One standard under development by the Institute for Electrical and Electronics Engineers (IEEE) is called 802.16 or “Air Interface for Fixed Broadband Wireless Access Systems” is closely related to the development of wireless MANs. An amendment to IEEE 802.16 called IEEE 802.16(e) covers “Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands”.
Multiple Input Multiple Output (MIMO) antenna systems are also being considered for incorporation into Wireless MANs. MIMO systems use multiple transmitting and multiple receiving antennas for communication of information. MIMO antenna systems allow spatial diversity. Spatial diversity that takes advantage of transmitting data from multiple sources that have a known physical spacing.
A typical cellular system that may form the basis for a wireless MAN includes a number of base stations controlled by a base station controller. The number of base stations define a corresponding number of cells. Within the cells, mobile terminals may roam and use a base station for wirelessly communicating with associated voice and data networks. In one case, the base station and mobile terminal communicate using time division multiplexing, wherein messages are exchanges formatted into frames. Portions of the frames may be defined as “channels” for carrying specific information.
As is known, a base station may initiate a transmission to a mobile terminal or a mobile terminal may initiate a transmission to a base station.
A random-access channel (RACH) is an uplink transport channel that may be used for initiating a transmission from a mobile terminal to a base station. After successful acquisition of a signal from a base station, the mobile terminal may read a number of parameters from a broadcast channel transmitted by the base station. To initiate a transmission to the base station, the mobile terminal first has to make itself known to the base station using a physical random access procedure.
The mobile terminal is unlikely to be able to accurately predict the transmission power that is required for a RACH transmission to be heard by the base station. As a result, the mobile terminal may, according to the physical random access procedure, transmit a so-called RACH preamble starting at low power. The mobile terminal may then increase the power level of the transmission of subsequent RACH preambles until the mobile terminal receives an acknowledgement of receipt of a RACH preamble from the base station. In the case of a positive acknowledgement, the mobile terminal transmits a RACH message part at the same power used for the most recent preamble transmission. The RACH message part may include an uplink resource request.
Additionally, the random-access channel may be used by a mobile terminal, or an SS, to transmit a “paging response”. When a base station initiates a transmission to a mobile terminal, the base station transmits a page to the mobile terminal. Upon receipt of a page, the mobile terminal performs the physical random access procedure, receives a positive acknowledgement, and transmits a RACH message part at the same power used for the most recent preamble transmission. The RACH message part, in this case, includes a paging response.
Notably, both uplink resource requests and paging responses are treated similarly. When a mobile terminal is initiating an uplink transmission or responding to a page, the mobile terminal may be required to perform the lengthy random access procedure.
For certain communications from mobile terminal to base station, there are alternatives to the lengthy random access procedure. One such alternative may be used by a mobile terminal to provide a location update to a base station.
Mobile terminals are known to have modes of operation, defined by standards (e.g., IEEE 802.16(e)) to which the operation of the mobile operation adheres. Among the modes of operation are an “Active” mode, wherein the mobile terminal is engaged in a bidirectional communication with a base station, and an “Idle” mode, wherein the mobile terminal does not have an immediate requirement for communication with a base station.
Ideally, a base station controller maintains general location information for a particular mobile terminal. If, for instance, a connection is to be established with the particular mobile terminal, the base station controller may initiate paging at each base station in one or more paging groups of base stations in the general location of the particular mobile terminal. In a first case, the particular mobile terminal frequently sends location update information to a proximate base station, which forwards the location information to the base station controller. The base station controller may then be very precise in instructing base stations (perhaps only the proximate base station) to send pages to the particular mobile terminal. In a second case, the particular mobile terminal seldom sends location update information to a proximate base station. The base station controller may then be required to instruct a great many base stations to attempt to send pages to the particular mobile terminal in order that a base station proximate to the particular mobile terminal may be so instructed. The first case has high accuracy at the expense of high overhead. The second case in very inaccurate, but the network is not clogged with mobile terminals reporting location updates.
Typically, a mobile terminal will transmit a location update triggered by movement from being within range of a base station in a first paging group to being within range of a base station in a second paging group.
The previously mentioned alternative communication method, used by a mobile terminal to provide a location update to a base station, involves a contention-based resource. From messages broadcast by a given base station, a mobile terminal may determine members of a pool of Pseudorandom Noise (PN) codes that may be used to encode a location update. The base station may select a PN code from the pool and encode a location update for the base station. Unfortunately, another mobile terminal in the same cell may simultaneously select the same PN code for encoding a location update. In such a case, neither location update is received and registered by the base station and it may be considered that a “collision” has occurred.
In addition to the active and idle modes of operation discussed above, a mobile terminal may also be in a “Sleep” mode and a “Normal” mode as defined in IEEE 802.16(e).
Many problems may be perceived associated with the current draft of the IEEE 802.16(e) standard.
For a first example, downlink or uplink transmission of a protocol data unit (PDU) within a listening window are performed in normal mode. Unfortunately, an uplink PDU received within a listening window cannot trigger a mode transition. That is to say, if a mobile terminal wants to return to normal mode from sleep mode, the mobile terminal will have to wait until a sleep window. Such waiting may be considered to result in an unnecessary delay.
For a second example, any downlink/uplink short data burst traffic to/from a mobile terminal in sleep mode must be sent during a listening window of the mobile terminal. It may be considered that, for applications with deterministic traffic patterns, there is room for improvement.
For a third example, any uplink PDU sent during a sleep-window, other than a RNG-REQ message and a DPC-REQ message, may be considered an indication of a mode transition of a mobile terminal in sleep mode. As such, an uplink short data burst must be either sent during a listening window, which may be considered to result in unnecessary delay, or sent after entering normal mode, which may be considered to cause unnecessary mode transmission overhead.
The current draft of the IEEE 802.16(e) standard does not support a downlink unicast short data burst to a mobile terminal that is in idle mode or an uplink short data burst from a mobile terminal that is in idle mode. It is considered that such support would be beneficial.